In order to achieve low noise in the receiver of a television set or communications equipment, an input signal is first amplified by a low-noise amplifier and then each is processed by following stage. Further, in the receiver of a television set, it is necessary to process a high amplitude signal. For this reason, low distortion is sought for the low-noise amplifier in addition to low noise. An example of such a low-noise amplifier is disclosed in Patent Document 1.
FIG. 3 is a circuit diagram of a preamplifier 40 described in Patent Document 1. The preamplifier 40 includes an active element 42 having a base 54 connected to an input terminal 44 of the preamplifier 40 via an input protecting circuit 60, an emitter 58 connected to a ground terminal 48 via an emitter resistor RE, and a collector 56 connected to a power supply terminal 50 via a collector resistor RL. Provided between the base 54 and the collector 56 is a first feedback loop 64 for feeding back the output of the collector 56 via an emitter follower circuit 70 and a feedback resistor 68. Provided between the emitter 58 and collector 56 is a second feedback loop 66 for feeding back the output of the collector 56 via an inverting amplifier 72.
In accordance with the preamplifier thus constructed, input impedance seen from the input terminal is lowered by the first feedback loop 64 and matching of characteristic impedance is facilitated. Further, owing to the second feedback loop 66, a large negative feedback is applied and distortion of the output waveform can be reduced even if the value of the emitter resistor RE is small.
Further, a feedback-type preamplifier referred to as a transimpedance preamplifier for opto-electronic conversion and amplification is described as related art in Patent Document 2. Specifically, a preamplifier having input means for inputting a current signal and amplifying means having transistors for amplifying the input current signal is described. This preamplifier includes a control-current source connected to the input means for shunting the input current signal, and current detecting means connected to the interior of the amplifying means, wherein the control-current source is controlled by the current detecting means. The control-current source is connected in parallel with the input terminal in order to bypass overcurrent. A current mirror circuit with respect to current that flows into an initial-stage transistor of the transimpedance section is constructed by the control-current source and a diode, which is a current detecting means. As a result, the preamplifier is capable of changing bypass current in accordance with the current waveform of the input signal.
[Patent Document 1]
Japanese Patent Kokai Publication No. JP-P2004-24309A
[Patent Document 2]
Japanese Patent Kokai Publication No. JP-A-9-130157